The increasing prevalence of digital communications systems has led to the widespread use of digital encryption by governments and other entities concerned with maintaining the security of sensitive data.
Generally speaking, an encryption system includes an encryption device that scrambles the value of each bit location in a data stream with respect to a particular encryption key or code. This has the effect of rendering the data stream unintelligible. A corresponding decryption device unscrambles the encrypted data stream by using a key that matches to the original key of the encrypting device. An example of a popularly used encryption device is the KG 81/94 family of key generators (KGs).
Unfortunately, such an encryption technique if applied alone to all data bits in a data stream prevents the use of widely popular and highly cost effective time division multiplexing (TDM) systems such as the T1 and T3 systems over many transmission networks. In a T1 system, 8 bit words from 24 channels, sampled at 8kHz, are multiplexed together to form the first 192 bits of a T1 frame. The 193rd bit in each frame is a T1 framing bit which is used to identify the end of the current frame and beginning of the subsequent frame in the bit stream. This approach requires that the T1 frames must be transmitted at 1.544 Mhz. The framing bits are utilized at a T1 receiver to define the end of the T1 frames and to provide for proper demultiplexing of channels within the frame. If the T1 formatted data were to be scrambled by an encryption device such as the one described above, then the T1 framing bit would also be encrypted. Thus the information contained in the framing bits would be unable to be utilized by a intermediate transmission network for frame synchronization and timing, and, as a consequence, could not be transmitted over such a T1 network.
One method for alleviating the problem discussed above is to use dedicated lines between communications sites rather than a shared transmission network as dedicated lines do not require intermediate synchronization. This approach suffers the disadvantage of decreasing the flexibility and increasing the cost of the overall communications system.
Another method of encrypting a T1 system is to sacrifice some of the usable bandwidth to provide additional room for some of the overhead functions such as additional (unencrypted) frame bits. This can be done by either reducing the number of available channels from 24 to 23 or by reducing bandwith of the 24 channels from 64 Kbps to 56 Kbps. This method of encrypting reduces the utility of T1 communications in sending video information and also increases the format sensitivity. With the bandwidth reduced to 56 Kbps, the T1 lines are not compatible with the integrated services digital network (ISDN) standards, since ISDN requires 64 Kbps bearer channels.
One prior art device or system for providing for encryption in a T1 communications system is disclosed in U.S. Pat. No. 4,866,773 (Lubarsky). The Lubarsky device provides for the removal of TDM framing bit information and the reinsertion of this information in a different location in the TDM bit stream. This method of relocating the TDM framing bit information is only effective with the particular type of encryption used in the Lubarsky device and would be ineffective with the type of encryption described above.